Refrigeration control system



REFRIGERATION CONTROL SYSTEM Filed Feb. 24. 1939 2 sneetsfsheet 1 April 29, 194.1. AQ B. NEWTON .REFRIGERATI-ON CONTROL SYSTEM Fired Feb. 24. 1939 2 neetss'neet 2 2. u.. w .H 8 Bmw... .l M m 9 :m lI-\ .I 9 mi2 9 9 2... a am a n E H n L WIG? q t@ 1 51 w.. l ma 1. '.9 f DI o 2 n w I. .l .r w .1 f F w convkesson y termined high value. rangement accurate temperature conditions are Patented Apr. .2.9, `1941 UNTLD .STATES5 'l PATENT oFFI-CE i REFRIGERATION CONTROL SYSTEM Alwin B. Newton', Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application February 24., 1939, Serial No. 258,229

(Cl. (i2-4) 12 Claims.

This invention relates to controlsystems for a refrigerating apparatus and particularly to a control. system wherein the refrigerating apparatus is placedin operation only when the thermostatic control means calls for cooling, when the suction pressure rises to a predetermined high value which may be a defrosting value and when thehead pressure decreases to a predetermined low value, and whereinv the refrigerating appara' tus is. maintained in operation until either the thermostatic control means is satisfied or the suction pressure decreases to a predetermined low value or the head pressure increases to a prede- With such a control armaintained, defrostingeach cycle of operation is provided, starting ,of the compressor against high high or when the suction pressure becomes'too low is provided.

When the above control system is applied to a multiple fixture refrigerating apparatus, the thermostatic control means usually includes a plurality of thermostats arranged in parallel, one

result that defrosting of the evaporators will not be accomplished.

It is therefore an object of this invention to avoid the above difiiculty by providing means for insuring a defrosting operation, In carrying out this feature of the invention a time op;- erated means is utilized for stopping operation of the compressor whereupon it cannot be restarted until all of the evaporators have defrosted. A further object is to have the timing cycle initiated upon starting of the compressor and if the compressor is not stopped by the control system within .the time cycle, then the timing means will stop the compressor at the end of the time cycle to cause defrosting. This will also tend to cause the thermostats to call for cooling and be satisfied together'so that. the control system will tend to operate in the -normal manner. 4In this way defrosting of the evaporators is assured.

When the compressor 4of the above outlined control system is located in a relatively. cold location, such as a basement or an outside shed, it

may well happen that in the winter-time the ambient temperature around the compressor and/or -the suction line will be so loW- that the suction pressure cannot rise to the predetermined high headpressure is prevented, and stopping of the compressor when the head pressure lbecomes too defrosting value and under these conditions the compressor could not be started. This-.may ocour regardless of whether the system be a single or multiple fixture systeml It is therefore anotherobject of this invention to avoid this diiiiculty by providing means for starting vthe operation of Vthe compressor independently of the value of the suction pressure.

In carrying out this feature of the invention a time operated means is utilized for starting operation of the compressor independently of the suction pressure and once the compressor has thus been operated, the heat from theA compressor and motor will usually increase the ambient temperature' suiiciently so that the system will then operate in-the normal manner. A further object of this invention is to have the time operated means start the compressor independently of the suction pressure and the thermostatic control means to maintain the ambient temperature surrounding the compressor su'ciently high at all times. Still another object of this invention, as

exemplified by a modification, is to have the time operated means start the compressor independ` ently of suction pressure only when the thermostatic control means is calling for cooling. Still another object of this invention is to place the compressor in operation by thetime operated means and to'continue the compressor in operation independently of the time operated means as long as the suction pressure is above the predetermined low value andthe control system is demanding operation of the compressor.

A further object of this invention is to provide a time operated means in conjunction with theA above outlined control system for accomplishing both operations, namely, interrupting operation of the compressor to assure defrosting and starting operation of the compressor to avoid dilliculties arising from a cold location of the compressor.

Still another object of this invention is to prevent starting of the compressor until the head pressure decreases to a predetermined low value regardless of whetherV the thermostatic control means or the time operated means demand starting of the compressor. In this manner starting of the compressor against'high head pressures is at all times prevented.

Other objects and advantages will become apparent to those skilled in the art upon reference to the accompanying specification, claims, and drawings, in which:

Figure 1 is a diagrammatic illustration of one form of this'invention as applied to a multiple xture refrigerating apparatus, wherein the time operated means operates to stop operation of the refrigerating apparatus to assure defrosting and operates 4to start the refrigerating apparatus independently of suction pressure and the therother form of this invention wherein the time operated means operates to start operation of the compressor independently of the suction pressure only when the thermostatic control means is calling for cooling.

For a more thorough understanding of this invention reference is made to Figure 1 which illustrates two fixtures I and II to be cooled. The fixtures Ill and Il may be spaces, walk-in boxes, soda fountains, storage compartments or any type of apparatus to be cooled. While only two fixtures are shown for purposes of illustration, any number may be utilized. The xtures'll) and Il are cooled by means of cooling coils in the form of evaporators I2 and I3, respectively. Refrigerant is circulated through the evaporators I2 andl3 by means of a refrigerating apparatus generally designatedat I4 which may comprise a compressor l5 and'A an electric motor I6 for driving the same. Compressed refrigerant ows from the compressor l5 through a high pressure line I1 to a condenser I8 and condensed refrigerant is collected in a receiver I9. Liquid refrigerant ows from the receiver I9 through a liquid line 20 to the evaporators I2 and I3 which are shown to be arranged in parallel. Evaporated refrigerant is withdrawn from the evaporators I2 and I3 through a suction line 2| by the compressor I5. Expansion valves 22 and 23 which may take the form of thermostatic expansion valves control the supply of refrigerant to the evaporators I2 and I3, respectively, in a conventional manner. Solenoid valves 24 and 25 also control the supply of refrigerant to the-evaporators I2 and I3, re-

spectively, the arrangement being such that when the solenoid valves are energized refrigerant is allowed to flow to the evaporators I2 and I3 and when the solenoid valves are deenergized the supply of refrigerant to the evaporators I2 and I3 is shut off. f

The solenoid valves 24 and 25 are controlled by thermostatic control means 28 and 29, respectively, responsive to the temperature within the fixtures I0 and II. The thermostaticcontrol means 28 and29 also operate in conjunction with a unitary control arrangement generally designated at 39 for controlling the operation of the compressor motor I6. A time operated means generally designated at 3I also operates in conjunction with the unitary control -arrangement 30 for controlling the operation of the compressor motor I6.

The thermostatic control means 28 may comprise a bellows 34 containing a volatile iiuid for operating a lever 35 against the action of an adjustable tension spring v36. The lever 35 in arrangement being such that when the -temperature within the nxture III rises to 42, illustratively, the switches 31 and 38 are closed and when the temperature decreases to illustratively, the mercury switches 31 and 38 are opened. The thermostatic'control means 29 responsive to the temperature within the iixture II may be exactly the same as the thermostatic control means 28 of the xture I9 and therefore like reference characters primed have been utilized for like parts.

The unitary control arrangement generally designated at 39 may be of the type shown and described in application Serial No. 196,447 led by Albert L. Judson and Carl G.' Krammer on March 17, 1938. For purposes of illustration in this application the unitary control arrangement is shown 'to comprise a control means generally designated at 40 responsive to variations in suction pressure, control means generally designated at 4I responsive to variations in head pressure, a relay or starter generally designated at 42, and an overload cutout mechanism generally designated at 43.

The suction pressure responsive control means may comprise a bellows connected by a pipe 46 to the suctionpressure line 2I for operating a lever 41 fulcrumed on a fulcrum member 48 against the action of an adjustable tension spring 49. One end of the tension spring 49 is connected to the lever 41 and the other end is secured to a nut 50 screw threadedly mounted on a screw 5I. pressure setting of this portion of the instrument may be adjusted at will. The lever 41 carries an insulating pad 52 upon which are mounted contacts 53 and 54. the contacts 53 and `54 being electrically .connected together. Thecontact 53 is adapted to engage a contact member 55 carried byl a. terminal 56 and the contact 54 is adapted to engage a contact member 51 carried by a terminal 56. Concentrically located cams 59 and 60 are independently rotatable for independently adjusting the positions of the contact members 55 and 51 with respect to the contacts 53 and 54. For purposes of illustration it is assumed that upon an increase in suction pressure the contact 53 iirst engages the contact member 55 at 15 lbs. andthen the contact 54 engages the contact member 51 at 32 lbs. which may be considered a defrosting value.

Conversely, upon a decrease in suction pressure.-

'secured to a nut 68 screw threadedly mounted on a screw 69. By rotating the screw 69 the tension of the spring 61 is varied and hence the pressure setting of this portion of thel instrument is adjusted at will.` 'Ihe lever 65 adjustably carries an abutment member 10 preferably made of insulating material and provided with abutments 1I and 12. The abutment 12 is adapted to engage a contact member 13 carried by a terminal 14 to move the contact member 13 out of engagement with a contact 15 vand the abutment 12 is adapted to engage a By rotating the screw 5I the vcontact member 16 carried by the terminal in suction pressure the contact member 16 isrst moved out of engagement with the contact 11 at 140|lbs. and then the contact member 13 is moved out of engagement with the con-` tact 15 at 180 lbs. `Upon a decrease in head pressure the contact member .13 first-,engages 4the contact 15 at 180 lbs. and then the contact member 16 engages the contact 11 at 140 lbs.

The relay or starter 42 may comprise an operating coil 19 for moving a bridge member 80 into engagement with contacts 8| and 82 and for moving a bridge memberv 83 into engagement with contacts 84 and 85 when energized.

this 'manner the thermostatic control means 28 and 29 open and close their respectivev solenoid valves 24 and 25 in accordance with temperature conditions in their associated iixtures- I0 and to maintain desired temperature conditions within these xtures.

Assumelnow that the temperature within the.

i xture |0 rises to 42 so that the mercury switch When the operating coil 19 is deenergized the bridge members 80 and 83 are moved out of engagement with their respective contacts by means of springs, gravity or other means, not shown. Y y

The overload cutout device, 43 vmay comprise a casing 81 to which is secured `a. terminal 88. A heater element 8-9 is connectedA between the terminal 88 and the contact -84 and when thecurrent flow through the heater element 89 be-. comes excessive due to an overload condition, a

thermostatic element trips a latch,. not shown,

for separating control contacts 90 and 9|. The control contacts 90 -and 9| 4may be reclosed by means of a manual reset lever 92.

The unitary control arrangement 30 is also provided with power terminals 94 and 95 to which are connected line wires91 and 98 vleading from some source of power, not shown. The unitary control arrangement minal 96.

The time operated means 3| may comprise a rotor |00 inuenced by a eld Winding'll connected by wires |02 and |03 acrossthe line wires 91 and 98. The rotor |00 is thereforemaintained in continuous operation and operates through a reduction gear train |04 for operating adjustable cams |05 and |06. Cam |05 operates 'a lever |01 which in turn operates a mercury switch |08 and cam' |06A operates a lever |09which in turn operates a mercury switch ||0. The mercury switch |08 is normally closed and the mercury switch ||0 is normally open. For purposes of illustration it is assumed that the cams |05 and |06 make one complete revolution in an hours time and that on the hour the switch |08 is moved to an open position .for an interval of one minute and on the half-hour the switch ||0 is moved to a closedposition for an interval of one minute. 1

APower is supplied to the solenoid valves 24 and 25 from line wires ||2 and ||3 leading from some source of power, not shown. When the temperature within the xture I0 rises to 42 the mercury switch 38 is closed to complete a circuit from the line wire ||2 through wire I4, mercury switch 38, wire ||5, solenoid valve 24 and wires ||6 and ||1 back to Nthe other line wire ||3. Completion of this circuit energizes the solenoid valve 24 to allow flow of refrigerant into the evaporator I2 andthe solenoid valve 24 remains energized until the temperature within the fixture |0 decreases to 40 at which time the mercury -switch 38 is opened. Solenoid valve 25 for the xture is controlled in exactly the .same manner as the solenoid valve 24 of xture Y terminal 94, wire |32, terminal 88, heater ele also includes a control ter- 31 is closed. that the suction pressure increases .to 32 lbs. indicating thatthe evaporators12'and I3 have defrosted to move the contacts 53 and 54 into engagement with the contact members 55 and 51, and that the head pressure has decreased rto 140 lbs. to cause the contact members 13 and 16 to engage thecontacts 15 and 11. A main starting circuit for the operating coil 19 of the relay or starter 42 is thereupon completed and this starting circuit may be traced from the line wire 91 through terminal 94, wire |20, contact 15, contact member 13, terminal 14, Wires |2|, |22 and |23, mercury switch 31, wires |24 and |25, normally closed mercury switch |08 of the time operated means3l, wire |26, terminal 56, contact member 55, contacts 53 and 54, contact members 51 and 16, contact 11, wire |21, contact 82, wire |28, control contacts 90 and 9|, wire |29, operatingcoil 19, Wire ment 89, contact 84, bridge ymember 83,-contact 8,5, wire |33,l compressor motor 6, wire |34, and` terminal95 back to the other line wire 98. Completion of vthis circuit `causes operation of the compressor I5 and hence circulation .of refrigerant through the evaporators I2 and I3.

Movement of the bridge member into engagement with the contacts 8| and 82 of the relay or starter 42 completes a main maintaining circuit for the operating coil 19 which is independent of contact 54, contact members 51 and 16 and contact 11. This maintaining Icircuit may be traced from the line wire 91 through terminal 94, wire |20, contact 15, contact member 13, terminal 14, wires '|2|, |22 and |23, mercury switch 31, wires |24 and |25l normally closed switch |08, wire |26, terminal 56, contact member 55, contact 53, wire |36, contact 8|,' bridge member 80, contact 82, wire |28, control contacts and 9|. wire |29, operating coil 19, wire |30 and terminal 95 back to the other line wire 98. Completion of this circuit maintains the compressor in operation until such time as either the temperature within the fixture |0 decreases to 40 or the suction pressure decreases to 15 lbs. or the head pressure increases to lbs. When any of these lcontingencies occur the compressoris stopped and cannot again be restarted until such time as the temperature within the xture rises to 42,

Y the suction pressure kincreases to at least 32 lbs.,

l connections have been utilized. It is here noted that the mercury switches 31 and 31' are located in parallel so that either thermostatic control means 28 or 29 may place the compressor in op eration and the compressor will be maintained l in operation until such time as both thermostatic control means 28 and 29 are satisfied.

Summing up, the control system thusl far described places the compressor in operation when either or both of the control means 28 and 29 call for cooling, when the suction pressure increases to 32 lbs. indicating that the evaporators I2 and I3. have defrosted, and when the head pressure is below 140 lbs. and continue the colmpressor in operation until either both, of the thermostatic control means 28 and 29 are satisfied or the suction pressure decreases to 15 lbs. or the head pressure increases to 180 lbs.

By reason of the parallel arrangement of the thermostatic control means any thermostatic control means may start operation of the com- `pressor and all must be satisfied before the compressor is stopped. It is therefore seen that a situation may arise where there will be at least one thermostatic control means calling for cooling at all times so that the compressor will remain in` continuous operation with the result that defrosting of the evaporators will not be accomplished.v This is particularly true where a large number of'xtures are cooled by a common compressor. It is here noted that the normally closed mercury switch |08 of the time operated control means 3| is included in series in both the main starting and main maintaining circuits outlined above and when this normally closed mercury switch |08 is opened for the one minute interval the relay or starter 42 is dropped out and the compressor is stopped. The compressor as pointed out above cannot again be restarted until such time as the evaporators I2 and I3 are defrosted. The normally closed mercury switch |08 of the time operated means 3| therefore operates to stop operation of the comall times assured.

When the compressorl of the above outlined control system isrlocated in a relatively cold location, such as a basement or an outside shed, it may well happen that in the winter-time the ambient temperature surrounding the compressor will be so low that the suction pressure cannot rise to the predetermined value of 32 lbs. which is a requisite for the starting of the compressor and under these conditions the compressor could not be started. The normally opened switch of the time operated means operates to start the compressor independently of the suction pressure responsive control means 40, this switch being closed for a period ofv one minute every hour.

Assume now that the ambient temperature surrounding the compressor is such that the suction p'resure can only rise to 25 lbs. whereupon the compressor cannot be placed in operation by the above outlined control system. When the mercury switch I|0 is moved to a closed position an auxiliary starting circuit is completed from the line wire 91 through terminal 94, wire |20, contact 15, contact member 13, terminal 14, wires |24 and |38, mercury switch ||0, wire |39, terminal '58, contact member 16, contact 11, wire |21, contact 82, wire |28, control contacts and 9|, wire |29, operating coil 19, wire |30, and terminal back to the other line wire 98. Completion of this auxiliary starting circuit pulls in 'the relay or starter 42 to place the compressor in operation and pulling in of the starter also completes an auxiliary maintaining 'circuit which may be traced from the line wire 91 through terminal 94, wire |20, contact 15, contact member 13, terminal 14, wires |2| and |38, mercury switch ||0, wire |40, control terminal 96, wire |4|, contact 8|, bridge member 80, contact 82, wirev |28, control contacts 90 and 9|, wire |29, operating coil 19, wire |30, and terminal 95 back to the other line wire 98. Completion of this auxiliary maintaining circuit maintains the operating coil energized as long as the mercury switch |I0 is closed and effectively prevents short-cycling of the compressor motor I6.

If as vassumed above the suction pressure could only rise to 25 lbs., the contact 53 and the contact member 55 would be closed and if any A of the thermostatic control means 28 and 29 were calling for cooling the relay or starter 42 would be maintained pulled in by the above outlined main maintaining circuit. Hence the compressor would remain in operation until either the thermostatic control means 28 and 29 become satised or the suction pressure decreases to 15 lbs. or the head pressure increases to lbs. By this time the compressor and the compressor motor would most likely heat the air surrounding the compressor so that the suction pressure could rise to the cut-in value of 32 lbs. Thereafter .the compressor would operate in the normal manner outlined above.

Accordingly, in Figure 1 in addition to the control sequence performed by the normal control system the time operated means 3| operates to interrupt operation of the compressor at denite intervals to assure that defrosting of the evaporators will occur and operates to start the compressor at intervals to avoid diculties arising from the location of the compressor in a cold place.

Figure 2 shows substantially the same control system as is illustrated in Figure 1 and like reference characters for like parts have been utilized in Figure 2. The main difference between the structure of Figure 2 and that of Figure 1 is that a different time operated means is utilized for interrupting operationof the compressor to assure defrosting. The time operated means of Figure 2 is generally designated at |50 and may comprise acasing |5| which may be made of heat insulating material. A thermostatic element |52 is secured to a bracket |53 carried by the casing |51 and carries a contact |54 which is adapted to engage an adjustable stationary contact |55. The adjustable stationary contact |55 is screw threadedly mounted in a post |56 and is provided with a. knob |51 for adjusting the position of the same. A heater element |58 located adjacent the bimetallic element |52 is utilized for heating the bimetallic element |52 to warp the same downwardly to move the contact |54 out of engagement withv |55. One end of the v wires |64 and |65, terminal 56, and so on 'I through the various contacts to complete the above outlined main Vstarting circuit. Completion of this main starting circuit pulls in the relay or starter 42 to operate the compressor and to complete a main maintaining circuit which may ibe traced --from the line wire 91 through terminal 94, wire |60, heater |58, bimetallic element |52, contacts |54 and |55, post |56, wire 6|, contact 15, contact member 13, terminal 14, wires |62 and |63, switch 31, wires |64 and |65, terminal 56 and so on through the various contacts of the unitary control arrangement. T'he compressor therefore remains in operation until either both of the thermostatic control means 28 and 29 are satised or until the suction pressure decreases to lbs. or until the head pressure increases to 180 lbs. When any of these contingencies occur the compressor is shut down and cannot again 'be restarted until the evapo. rators have defrosted to allow the suction pres- -sure to rise to 32 lbs.

When the compressor is placed in operation in the manner outlined above'the heater element |58 is energized since this heater element is .included in both .the main starting and main maintaining circuits for the relay or starter 42. This heater element |56 then tends to heat the bimetallic element |52 to dex the same downwardly and after a time interval depending upon the adjustment oi' the contact |55, the conv-tacts |54 and |55 are separated to break the above main starting and main maintaining circuits to drop out the relay or starter 42 and'f hence stop operation of the compressor. For purposes of illustration, it is assumed that it takes substantially one hour for the heater |58 to heat the bimetallic element |52 sufficiently to separate the contact |54 from the contact |55. By reason of the control arrangement of Figure 2, if the normal operation oi! the refrigerating apparatus fails to stop the compressor within a predetermined time interval as determined by the time operated ,means |50, the time operated means will stop operation of the compressor at the termination of this time interval whereupon defrosting of the evaporators is assured. In other words, the time operated" means |50 of Figure 2 limits the length of the ,on periods oi. the compressor so that defrosting of the evaporators must occur after` the compres- Asor has run a predetermined length of time.

from that of Figure 1 in that in, Figure 2 the` timing cycle begins as soon as the compressor is placed in operation while in Figure 1 the times at which the time operated means 3| stops operation of the compressor occur periodically without regard to the starting of the compressor.

Figure 3 discloses acontrol arrangement which is substantially the same as that of Figure 1 with the exception that a different time operated means is utilized for starting operation of the compressor independently of the suction pressure responsive control means. Like parts in F1gures l and 3 have been designated by like reference characters. The time operated means of Figure Accordingly, the operation of Figure 2 differs wires |80 a casing |10 in which is mounted a rotor |1| iniluenced by a ileld winding |12. The rotor |1| yoperates through a reduction gear train |13 an adjustable cam |14 which in turn ver |15. The lever |15 operates a |16 which is normally open, the switch being zllosed for an interval say of one minute every our.

Assume now that the temperature within the xture I0 rises to 42, that the suction pressure rises to 32 lbs. and that the head pressure decreases .to 1,40 lbs. When this occurs a main starting circuit is completed from the line wire 91 through terminal 94, wire |20, contact 15, contact member 13, terminal 14, wires |80 and |8|, mercury switch 31, wires |82, |83 and |84, termi nal 56 and through the various contacts and contact members to pull in the relay or starter 4 2. Pulling in of the relay or starter 42 compressor and completes a main maintaining circuit in the manner pointed out above. Accordingly, in Figure 3 the compressor is placed in operation when either of the thermostatic control means call for cooling, the suction pressure rises to 32 lbs. and the head pressure decreases to 140 lbs. and is continued in operation until either all of the Vthermostatic control means are satised, or the suction pressure decreases to 15 lbs. or the head pressure increases to 180 lbs. Thus far Figure 3 operates in exactly the same manner as Figures 1 and 2.

Assume now that the xture |0 increases to 42, that the head pressure decreases'to 140 lbs. and that the ambient temperature surrounding the compressor is such that thesuction pressure cannot rise to the starting value of 32 lbs. but can only rise to say 25 lbs. Under .these conditions a circuit is completed from the line wire 91 through `terminal 98, wire |20, contact 15, contact member 13, terminal 1d, and 8| ,mercury switch 31, wires |82, |83, |86 and |81, field Winding |12 of the time operated device |69, and wires |88 and |89 back to the line wire 98. Completion oi" this circuit operates the time operated means |69 and when the time operated means has operated sufficiently far to close the normally opened switch |16, an auxiliary starting circuit is completed from the line wire 91 through wire |20, contact 15, contact member 13, terminal 14, wires |80 and |8|, mercury switch 31, wires |82, 83, |86 and |90, mercury switch |16, Wires |9| and |92, terminal 59, contact mem-ber 16, contact 11, wire |21, contact 82, wire |28, control. contacts 90 and 9|, wire |29, operating coil 19, wire |30 and terminal 95 back to the other line wire 98. Completion of this auxiliary starting circuit pulls in the relay or starter 42 to operate theV compressor i6. -Pulling in of the relay or starter 42 also completes the main maintaining circuit which is independoperates a le- 4 ent of the time operated means |69 since the suction pressure at this time is assumed to be 25 lbs. Therefore the compressorisy maintained in operation until'the thermostatic control means are satisfied or the suction pressure decreases to o temperature of the air surroundingk the comi the normally open switch pressor so that thereafter the compressorwill operatein the normal manner outlined above. It is here notedthat the-switches 31 of the thermostatic control means are located in series with |16 of the time operated means |69 so that the-time operated means mercury switch operates the temperature within the frigerant therethrough, a

can place the compressor in operation only when one of the thermostatic control means is calling for cooling. In this respectl the sequence of operation performed by the' structure of Figure 3 is different from that of Figure 1.

From the above it is seen that I have provided a time operated means operating in conjunction with a defrosting control system for accomplishing two improved modes of operation, namely, interrupting operation of the compressor to assure defrosting, and starting operation of the compressor independently of suction pressure to avoid diiculties arising from a cold location of the compressor. Although for purposes of illustration various temperature and pressure values have been set forth, it is obvious that other values may be utilized within the confines of this invention. While several forms of this invention have been disclosed, other forms thereof may become apparent to those skilled in the art upon reference to this disclosure and therefore this invention is to be limited only by the scope of the appended claims and prior art.

I claim as my invention:

1. In a control system for a multiple fixture refrigerating apparatus including an evaporator for each f'lxturefor cooling the same and a compressor for circulating refrigerant through the evaporators, the combination` of, a valve for each evaporator for controlling the circulation of refrigerant therethrough, a thermostat in each fixture for operating its associated valve upon a call for cooling to allow circulation of refrigerant through the evaporator of that fixture, a pair of sequentially operated switches, means responsive to an increase in suction pressure for sequentially closing said switches, a normallyclosed switch, a starter for the compressor including an operating coil and a maintaining switch which is closed upon energization of the operating coil, means for `completing a starting circuit for the operating coil through the thermostats in parallel, the sequentially operated switches in series and the normally closed switch to energize the starter and start the compressor when any thermostat calls for cooling providing the suction pressure has risen to a predetermined high defrosting value, means for completing a maintaining circuit for the operating coil through the thermostats in parallel, the first to close of the sequentially operated switches, the normally closed switch and the maintaining switch to maintain the starter energized and the compressor in operation until all of the thermostats are satisfied or the suction pressure decreases to a predetermined low value, and timing means for opening the normally closed switch to deengize the operating coil and Q whereby defrosting of the evaporators of alla` the fixtures is assured.

2. In a control system for a multiple xture refrigerating apparatus including an evaporator for each fixture for cooling the same and a compressor for'circulatlng refrigerant through the evaporators, thecombination of, a valve for each evaporator forg.cntrolling the circulation of rethermostat in each fixture for operating its associated valve upon a call for cooling to allow circulation of refrigerant through the evaporator of that fixture, a pair of sequentially operated switches, means responsive to an increase in suction pressure for sequentially closing said switches, a normally closed switch, a starter for the compressor including an operating coil and a maintaining switch which is closed stop the compressor?.

tially operated switches, the normally closedA switch and the maintaining switch to maintain the starter energized and the compressor in operation until all of the thermostats are satisfied or the suction pressure decreases to a predetermined low value, and timing means placed in operation upon energization of said starter for opening said normally closed switch a predetermined time thereafter for deenergizing said starter to stop operation of the compressor whereby defrosting of the evaporators of all of the fixtures is assured.

3. In a control system for a multiple fixture refrigerating apparatus including an evaporator for each fixture for cooling the same and a compressor for circulating refrigerant through the evaporators, the combination of, thermostatic means responsive to the temperature of each fixture; control means responsive to variations in suction pressure, means controlled by the thermostatic means and thecontrol means to start the compressor only when any of the thermostatic means calls for cooling and the suction pressure has increased to a defrosting value and timing means placed in operation upon starting of the compressor for stopping operation of the 1Acompressor a predetermined time thereafter whereby defrosting of the evaporators of all of the fixtures is assured.

4. In a control for a multiple fixture refrigerating apparatus including an evaporator for each fixture for cooling the same and a compressor for circulating refrigerant through the evaporators, the combination of, thermostatic means responsive to the temperature of each xture, valve means associated with each xture and controlled by its associated thermostatic means to allow circulation of refrigerant through the evaporator of that fixture upon a call for cooling, control means responsive to variations in suction pressure, means controlled by the thermostatic means and the control means to start the compressor only when any of these thermostatic means calls for cooling and the suction pressure has increased to a defrosting value and timing means placed in operation `-uponstarting of the compressor for stopping operation'fof the compressor a predetermined timethereafter whereby. defrosting of the evaporatorsof all of the fixtures is assured.y

5. In a control system for fa refrigerating aptemperature condition of the medium being-controlledv by theV evaporator means, means responsive to changes in suction pressure, control 'means controlled by the thermostatic means and the suction pressure responsive means for starting operation of the compressor 'upon a call for cool- .ing by the thermostatic means provided the suction pressure has risen Vto a predetermined high value, and time operated means for controlling said control means to start operation of the compressor independently of the suction pressure responsive means whereby-the compressor may be started even though thesuction pressure has not risen to the predetermined value.

6. In a control means for a refrigerating apparatus having evaporator means for controlling the temperature condition of a medium and a compressor for circulating refrigerant through the, evaporator .-means, the combination of, thermostatic means responsive to changes in the temperature condition of the medium being controlled by the evaporator means, means responsive vto changesI in suction pressure, control means controlled by the thermostatic means and' the suction pressure responsive means for starting operation of the compressor upon a call for cooling by the thermostatic means'provided the suction pressure `has risen to a predeterminedhigh value, and time operated means for controlling said control means to start operation of the compressor at definite intervals independently of the thermostatic means and the suction pressure responsive means to increase the ambient temperature of the medium surrounding the compressor whereby the suction pressure'is increased to a value which allows starting of the compressor by the suction pressure responsive means and the thermostatic means.

7. In a control system for a refrigerating apparatus having evaporator means for controlling the temperature condition of a medium and a compressor for circulating refrigerant through the evaporator means, the combination of., thermostatic means responsive to changes in the temperature condition of the medium being controlled by the evaporator means, means. responsive to changes in suction pressure, control means controlled by the thermostatic means and the suction pressure responsive means for starting operation of the compressor upon a call for cooling by the thermostatic means provided the suc- 8. In a control system fora refrigerating apparatus having evaporator means for controlling the temperature condition of a medium and a compressor for circulating refrigerant through the evaporator means, the combination of, thermostatic means responsive to changes in the temperature condition of the medium being controlled by the evaporator means, means responsive to changes in suction pressure, control means controlled by the thermostatic means and the suction pressure responsive means for starting operation of the compressor upon a call for cooling by the thermostatic means provided the suction pressure has risen to a predetermined high value and to continue the compressor in operation until either the thermostatic means is satisi'led or the suction pressure decreases toa predetermined low value,`and time operated means for controlling said control means to start operation o the compressor at definite intervals independently of the thermostatic means and the suction pressure responsive means to increase theV "mbient temperature of the medium surrounding refrigerating apparatus including an evaporator foreach iixture for cooling the same and a compressor for circulating refrigerant through the evaporators, the combination of, thermostatic means responsive to the temperature of each iixture, control means responsive to variations in suction pressure, means controlled by the thermostatic means and the control meansv to startv the compressor when any .of the thermostatic means calls for cooling and the suction pressure hasincreased to a defrosting value and to continue the compressor in operation until all of the thermostatic means are satised, time operated means for stopping operation of the compressor whereby defrosting of al1 of the evaporators is assured, and time operated means for starting operation of thecompressor independently of the suction pressure responsive control means `wherebythe compressormay be started eventhough the suction pressure has not risen to the predetermined value.

10. In va control system for a multiple xture refrigerating apparatus including an evaporator for each fixture for cooling the same and a compressor for circulating refrigerant through the Tevaporators, the combination of, thermostatic means responsive to the temperature of each xture, control means responsive to variationsin suction pressure, means controlledby the thermostatic means and the control means to start the compressor when any of the thermostatic means calls for cooling land the suction pressure has increased to a defrosting value and to continue the compressor in operation-until all of the thermostatic means are satlsed, time operated means for stopping operation of the compressor at deiinite intervals whereby defrosting of all of the evaporatorsis assured, and time operated means for starting operation of the compressor at denite intervals independently of the thermostatic means and the suction pressure responsive means to increase the ambient temperature surrounding the compressor whereby the suction pressure is increased to a value which allows starting of the compressor by the suction pressureresponsive means and the thermostaticl means. A

. 11. In a control system for a-muliple xture refrigerating apparatus including an evaporator for each fixture for cooling the same and a com.- pressor for circulating refrigerant through the evaporators, the combination of, thermostatic means responsive to the temperature of each iixture, control means responsive to variations in suction pressure, control means responsive to variationsv in head pressure, means controlled by the thermostatic means and the control means to start the compressor only when any of the thermostatic means calls for cooling, the suction pressure increases to a defrosting value and the head pressure decreases to a predetermined low value. and to continue the compressor in opera tion until either al1 of the thermostatic means are satised or the suction pressure decreases to a predetermined low value `or the head pressure increases to a predetermined high value, and'tim- .ing means placed in operation upon starting of whereby defrostlng of the evaporators of all of the fixtures is assured.

12. In a control system for a refrigerating apparatus having evaporator means-for controlling the temperature condition of a medium and a compressor for circulating refrigerant through the evaporator means, the combination of, ther- 'rnostatic means responsive to changes in the temperature condition of the medium being controlled by the Aevaporator means, means responsive to changes in suction pressure, means responsive to changes in head pressure, control means controlled by the thermostatic means the suction pressure responsive meansl and the head pressure responsive means to start operation of the compressor only when the thermostatic means calls for cooling, the suction pressure increases to a predetermined high value and the head pressure decreases to a predetermined 10W value and to continue the compressor in operation until either the thermostatic means is satisfied or the suction pressure decreases to a prede- 

